Anti-reflective (AR) coated substrates find use in many different applications, like cover sheets for framing art and photos, tv screens and other displays, lighting, architectural and horticultural glass, cover sheets for solar panels, etc. Numerous documents describe different processes for depositing an (AR) layer on a transparent substrate, applying various coating compositions. Especially for glass substrates, such processes often use sol-gel technology to provide a porous inorganic layer of about 80-200 nm thickness, that is about one-quarter of the wavelength of visible light. A single layer of such thickness and having a refractive index between that of the substrate (about 1.5 for glass) and air (about 1) will reduce the amount of light reflected from the substrate surface. Such anti-reflective layers may be applied to one or both sides of the substrate. An advantage of porous coating is that a single layer may suffice for reducing reflectance. In case of alternate techniques like sputtering or non-porous sol-gel layers, light reflection can be further decreased by applying multiple layers having different refractive indices.
A sol-gel process, also known as chemical solution deposition, is a wet chemical technique that is typically used for making a (porous) metal oxide layer starting from a solution or colloidal dispersion of an inorganic oxide precursor in a solvent (also called sol), which acts as precursor for an integrated network (or gel) of either discrete particles or network polymers. In such process, the sol gradually evolves to a gel-like diphasic system containing both a liquid and solid phase. Evaporation of at least part of the solvent (drying) is generally accompanied by shrinkage and densification, and affects final microstructure and porosity. Afterwards, a thermal treatment at elevated temperature is often needed to remove remaining solvent and other organics, and enhance further condensation reactions (curing) to result in mechanical and structural stability. Typical inorganic oxide precursors are metal alkoxides and metal salts, which undergo various forms of hydrolysis and condensation reactions. To increase porosity and pore size, pore forming agents may be added (in addition to solvent). In processes for making an anti-reflective layer on a substrate generally coating compositions are applied that have very low viscosity and comprise only low amounts of components that will form the final solid layer, e.g. a solids content of up to about 10 mass %. This means that in the sol-gel process significant amounts of fluid (solvent) need to be removed, which can lead to various problems; including premature evaporation.
In US2008/0241373A1 a process for making an AR coating on a substrate is described, comprising the steps of a) providing a liquid coating composition comprising a silica precursor, a polymeric glycol as pore forming agent, and at least two alcohols having different boiling points; b) applying the coating composition to the substrate; c) evaporating the alcohols from the applied layer; and d) heat curing the coating layer. It is indicated that by using a mixture of alcohols with different boiling points, solvent evaporation can be better controlled leading to improved process and product consistency.
Various known deposition techniques can be used to apply an AR layer on a substrate; including methods like spin-coating, dip-coating, spray-coating, roll-coating, slot die-coating, and the like. Both coating technique and coating composition can be fine tuned to reduce surface and other defects, and to result in a homogeneous layer of uniform thickness and properties.
A typical defect frequently observed in AR coatings is the appearance of a band along the edges of the substrate, especially in case of a flat or planar substrate like a glass or plastic plate or sheet, which band is visible as showing deviating light reflection and/or having a different hue than the remainder of the substrate's surface. In the text book “Sol-Gel Technologies for Glass Producers and Users”, M. A. Aegerter and M. Mennig (Eds.), 2004, ISBN: 978-1-4020-7938-2, p. 44, it is indicated that such edge defects are process-related, and appear in a band of about 8-20 mm from the edge. This publication makes specific reference to a dip-coating process, but similar edge defects are also observed with other deposition processes. Although such edge defects appear to be common, only few publications address reducing or preventing them. Occurrence of such defects, however, does pose a problem in practice, for example in cover glass for art or picture framing, or for solar panels. Currently, it is not uncommon to cut away the edges having deviating optical properties. This is not possible, however, in case of thermally tempered glass.
There is thus a need in industry for an improved process for depositing an anti-reflective layer on a transparent flat substrate, which process results in a substrate showing a reduced area at the edges having defects, or showing less visible edge defects.
It is therefore an objective of the present invention to provide such an improved process.
The solution to the above problem is achieved by providing the process as described herein below and as characterized in the claims.
Accordingly, the present invention provides a process for depositing an anti-reflective layer on a transparent flat substrate comprising the steps of    a) providing a liquid coating composition comprising at least one solvent, at least one inorganic oxide precursor, and at least one pore forming agent;    b) applying the coating composition to the substrate;    c) drying the applied coating layer; and    d) curing the coating layer to form an anti-reflective layer;wherein during drying step c) a gas flow is provided to the substrate at a flow rate of between 0.2 and 6 m/s.